Highly Asymmetric Optical Properties of β‑Ga2O3 as Probed by Linear and Nonlinear Optical Excitation Spectroscopy

β-Ga2O3 is a highly promising semiconductor for a deep ultraviolet (UV) emitter owing to its wide band gap, which significantly varies in the range of 4.49–4.74 eV because of its optical trirefringence in the monoclinic crystal structure (C 2h ). However, dominant photoluminescence (PL) emissions occur far below the band gap. These PL peaks at 3.0 and 3.5 eV were investigated by employing photoluminescence excitation spectroscopy. Intriguingly, the PL was found to be highly polarized along the (102) direction under one-photon absorption, two-photon absorption, and three-photon absorption, which is consistent with selection rules for optical transitions. Moreover, the measured nonlinear optical absorption coefficients are several times higher than the theoretical prediction, indicating that β-Ga2O3 possesses excellent optical nonlinearity with high polarization contrast. The absorption power dependence indicates that the PL arises from excitons bound to Ga vacancies (3.0 eV) and O vacancies (3.5 eV). This was also clearly supported by the depth-dependent PL shape as probed by a two-photon depth scan. A broad set of highly asymmetric optical properties clarified in this work are critical for the understanding of this wide-gap semiconductor and its potential use for UV sources and nonlinear optics especially when band-edge emission is realized.